There are various types of sounds adjacent to a vehicle driving on a road. If a driver is an aged person having at least partially impaired hearing or a person who has a bad sense of sound direction, he or she would be insensitive to respond to a particular sound made by a warning horn, a siren or the like, which he or she should pay attention to. In addition, since an in-vehicle technology regarding sound insulation has been developed, it is likely that even a driver having great hearing could not correctly hear a noise/sound occurred outside a vehicle. It might be driver's safety at risk that the driver does not keep eyes forward when he or she listens to a specific sound from the rear of the vehicle.
Accordingly, it might be necessary that information about a particular sound, such as what kind of sounds exists adjacent to a vehicle, which direction a sound comes from, and the like, is informed to a driver without disruptions to driving safety. Particularly, when plural vehicles may be operating near a subject vehicle, a driving safety would be enhanced if there are notifications or alerts regarding the plural vehicle.